1. Field of the Invention
This invention relates to consumable electrode arc welding for joining a gap having a V- or L-shape or a similar shape by weaving a torch along a welding line, and more particularly to a method for automatically controlling the center position of weaving to trace a weld line.
2. Description of the Prior Art
Automation of arc welding has been a subject of energetic studies in order to improve the productivity of welded structural materials, reduce the welding cost and make uniform the quality of welding operations. The greatest problem which is involved in the automation of arc welding is the control of the weaving path along the welding line.
In short, the welding line tracing operation depends on how well a welding line detector can overcome the structural limitations of welding materials and variations in the shape of the welding gaps and whether or not it can accurately detect a welding line to be followed in spite of deviations due to distortions during a welding operation. Therefore, there have thus far been proposed various kinds of detection systems, including mechanical, electrical and fluid type detection systems.
The conventional detection systems have a common difficulty in that it is difficult to attach a detector in a position close to a welding torch due to bad environmental conditions including the heat peculiar to arc welding, blazing light, fumes, spatters and narrow space. Therefore, in some cases the detector can be an obstacle and restricts application to certain structural materials, which is another problem in addition to the difficulty of detecting the welding line at an actual arc generating point.
In an attempt to overcome these problems or difficulties, many efforts have been made in the past several years to derive a method for detecting the weld line at an arc generating point. For example, studies are actually being made on a method of analyzing a picture image of the arc generating point taken by an ITV camera for the detection of the welding line, which however still has a number of technical and costwise problems to be solved before application, such as the limitations of the mounting position due to the use of a bulky ITV camera, costly equipments, etc.
On the other hand, there has been proposed a more acceptable method which, for real time detection of the welding line at an arc generating point, utilizes certain phenomena which take place during the welding operation. This method detects the welding line based on a of correlation between the length of welding wire extension and the amount of welding current without necessitating a detector exclusively for this purpose.
The present invention concerns an automatic weaving path control method of the category which utilizes such phenomena in welding operation.
A method of automatic weaving path control based on this sort of phenomena is proposed, for instance, in Laid-Open Japanese Patent Application No. 52-78654 in which the values of welding current at the reversing ends of a weaving motion are detected and compared with each other, and the position of a holder of a weaving source or the axis of the weaving motion is adjusted such that the currents at the reversing ends are equalized.
In the method mentioned above, the detected values of welding current at the opposite reversing ends of the weaving motion are compared with each other and a value of differential current which is obtained by subtracting a value of current at a left end position from a value at an opposite right end position is relied on as a value which represents a deviation (a deviate distance) of the center or median point of weaving from the weld line, so that the welding line tracing accuracy depends on the degree of accuracy with which the value of differential current/deviation is detected. Therefore, it becomes necessary to ensure detection of very small values of differential current, that is to say, to ensure extremely high detection sensitivity to the welding current.
Besides, in order to cope with alterations of welding conditions, it is required to secure high detection sensitivity over a wide range of welding current settings. For instance, the value of differential current/deviation is very small in the fillet welding of a short leg length where the welding is performed at a low current and with a small weaving width.
According to the results of experiments, for example, of a horizontal fillet welding of a leg length of 6 mm, using a shielding gas composition of Ar+20% CO.sub.2, a wire of a diameter of 1.2 mm, an average welding current of 280 A, a voltage of 28 V, a welding speed of 55 cm/min, a weaving motion of 150 cycles/minute, and a weaving width of 3 mm, the differential current as measured by an optical electromagnetic oscillograph is as small as about 5 amperes when the centerline of weaving is deviated by 1 mm toward a vertical plate. In this instance, if the target of tracing accuracy is set at 0.2 mm, there will be required a current detection sensitivity of 1/280, which is only possible by a complicated and costly control device.